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Accounting Changes - Narrative (Details) - CAD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2020 | Jun. 30, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Assets |  |  |  | $ 9,747 |  | $ 9,747 | $ 9,508 |
Depreciation and amortization |  |  |  |  |  | $ 654 | 590 | $ 574
Impairment loss | $ 70 |  |  |  |  |  |  |
Coal generation | Maximum |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Useful life measured as period of time, property, plant and equipment |  |  |  |  |  | 29 years |  |
Coal generation | Minimum |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Useful life measured as period of time, property, plant and equipment |  |  |  |  |  | 1 year |  |
Wind and Solar |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Useful life measured as period of time, property, plant and equipment |  |  |  |  | 30 years |  |  |
Wind and Solar | Maximum |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Useful life measured as period of time, property, plant and equipment |  | 15 years |  |  |  |  |  |
Wind and Solar | Minimum |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Useful life measured as period of time, property, plant and equipment |  | 10 years |  |  |  |  |  |
Change in Useful Lives Estimate | Sheerness |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Depreciation and amortization |  |  |  |  |  |  | (8) |
Change in Useful Lives Estimate | Coal generation |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Depreciation and amortization |  |  |  |  |  |  | 16 |
Change in Useful Lives Estimate | Wind and Solar |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Depreciation and amortization |  |  |  |  |  |  | 11 |
Decommissioning and restoration | Centralia Coal Mine |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Provision for decommissioning, restoration and rehabilitation costs |  |  |  |  |  |  | 141 |
Impairment loss |  |  |  |  |  |  | $ 141 |
Alberta Mine | Mining assets | Gross carrying amount |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Assets |  |  |  | 373 |  | $ 373 |  |
Alberta Mine | Change in Useful Lives Estimate |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Depreciation and amortization |  |  |  |  |  |  |  | $ 38
Alberta Mine | Change in Useful Lives Estimate | Mining assets |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Depreciation and amortization |  |  |  | 15 |  |  |  |
Alberta Mine | Decommissioning and restoration |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Provision for decommissioning, restoration and rehabilitation costs | 75 |  |  |  |  |  |  |
Sundance Unit 2 |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Impairment loss |  |  | $ 38 |  |  |  |  |
Sundance Unit 2 | Change in Useful Lives Estimate |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Impairment loss |  |  | $ 38 |  |  |  |  |
Sundance Unit 3 | Change in Useful Lives Estimate |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Impairment loss | $ 70 |  |  |  |  |  |  |
Sarnia | Decommissioning and restoration |  |  |  |  |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |  |  |  |  |
Provision for decommissioning, restoration and rehabilitation costs |  |  |  | $ 15 |  | $ 15 |  |

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